Radio frequency identification (RFID) is a technology that incorporates the use of electromagnetic or electrostatic coupling in the radio frequency (RF) portion of the electromagnetic spectrum to uniquely identify an object, animal, or person. RFID is coming into increasing use in industry as an alternative to the bar code. The advantage of RFID is that it does not require direct contact or line-of-sight scanning. RFID is sometimes also called dedicated short range communication (DSRC).
RFID tags can be passive or active. Active tags are typically powered by an energy source, such as an internal battery. Passive tags depend on an external power source to function.
A reader interrogates each tag to learn the tag's identity, uploads information from that tag, and then repeats the process with each of the remaining tags in sequence. Traditionally, in reading a group of tags, a reader communicates with tags one at a time so that the receiver within the reader is not confused by mixed data from many tags at the same time. The method of resolving individual tag data from a population of multiple tags is called anti-collision. Existing anti-collision methods generally suffer from slow multiple tag reading rates, and read rates can be severely reduced with large tag populations.
RFID tags can be attached to items in order to identify them for a variety of purposes. For example, RFID systems utilizing readers and tags can be used in a store environment to identify individual items for purposes of inventory control and purchasing transactions. In a typical store that has several readers spread throughout the store, the operational regions of readers can overlap and tags can communicate with several readers simultaneously.
The tag circuitry typically requires multiple clock cycles in order to perform any given function. The actual clock frequency and the number of clock cycles required for a function to be performed determines the data rate at which the tag can receive or transmit information. If the clock frequency is too low for the desired data rates, “one shots” and complex timing circuits are required to control the ordering of events within the tag.
One problem inherent in the prior art is power constraint. Passive tags have no battery and are powered by the RF energy transmitted by the reader. The more power a tag consumes, the closer the tag needs to be to a reader in order to operate. A corollary is that the less power or voltage a tag requires to operate, the farther a tag can be from a reader and still operate. Also, as the input voltage decreases, propagation delays within the tag circuitry increases, thereby reducing the operational speed of the tag. In many cases, the reduction in speed will cause the tag to operate improperly or not at all.
It follows that the number of tags that can be interrogated in a unit of time increases as the power increases. It also follows that the range at which a tag can be interrogated increases as the power required to operate that tag decreases.
Another problem arises in a typical store that might have several readers and several overlapping areas of interrogation. In this scenario, when tags are interrogated only one at a time and the entire bandwidth is occupied while a tag is being interrogated, only one reader and one tag can communicate at a given time in a given bandwidth.
It would therefore be advantageous to prevent, or at least limit, interference among multiple tags and/or multiple readers. It would also be desirable that the read speed (or read rate) of a multi-tag system be as high as possible.